Throughout this specification the use of the word “inventor” in singular form may be taken as reference to one (singular) inventor or more than one (plural) inventor of the present invention.
It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain prior or related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.
A metal detector in its simplest form may comprise an oscillating electrical supply producing an alternating current that passes through a coil, which in turn emanates an alternating magnetic field that is used to interrogate a physical region of space into which the alternating magnetic field emanates. If a piece of electrically conductive metal is proximate the coil, eddy currents are induced in the metal by the alternating magnetic field and this produces an alternating magnetic field of its own. The coil, or in some forms a second coil, of the metal detector may then measure this magnetic field emanating from the metal object by way of acting as a magnetometer. Accordingly, the change in the recorded magnetic field due to the metallic object can be detected.
The modern development of the metal detector began in the 1920s where an employee of Federal Telegraph Company in California, Gerhard Fisher, had developed a system of radio direction-finding, which was to be used for accurate navigation. The system worked extremely well, but Fisher noticed that there were anomalies in areas where the terrain contained ore-bearing rocks. He reasoned that if a radio beam could be distorted by metal, then it should be possible to design a machine which would detect metal using a search coil adapted for resonating at a radio frequency. In 1925 he applied for, and was granted, what was considered the first patent for a metal detector1. However, during the early years of World War II, Lieutenant Jozef Stanislaw Kosacki, a Polish officer attached to a unit stationed in St Andrews, Fife, Scotland, refined the design into a practical detector that became known as the Polish mine detector2. These early military metal detectors were heavy, ran on vacuum tubes, and needed separate battery packs. 1http://en.wikipedia.org/wiki/Metal_detector#Industrial_metal_detectors2Tadeusz Modelski (1986). The Polish Contribution to The Ultimate Allied Victory in The Second World War. Worthing, England. p. 221
Pursuant to the early developments of metal detectors, the most notable being the Polish mine detector, the first industrial metal detectors were developed in the 1960s and these found extensive use for mining and other industrial applications. Typical uses included de-mining (the detection of land mines), the detection of weapons such as knives and guns particularly in airport security, geophysical prospecting, archaeology and treasure hunting for hobbyists. In some industrial applications, metal detectors are also used to detect foreign bodies in food, and, as a further example, in the construction industry to detect steel reinforcing bars in concrete and for detecting pipes and wires buried in walls and floors.
Metal detectors have since been developed to provide greater sensitivity, lower noise and greater depth penetration by exploiting the characteristic electrical properties of materials, for example gold or iron, that differentiate them from the materials they may be situated in and, also by exploiting electrical signal properties, for example by way of synchronous mains rejection and so on. Examples of such developments are disclosed in U.S. Pat. No. 5,506,606 in the name of Candy and in Australian Innovation Patent No. 2010101019 in the name of Rockey. However, neither of these prior art references or any other prior or related art disclosures known to the inventor address the following problems with metal detectors.
A metal detector cannot be operated in close proximity with another metal detector because the electro-magnetic field generated from each unit will inevitably produce interference effects with the signal(s) received by the receiver circuitry of other metal detectors in range of the generated electro-magnetic field. By way of explanation, a metal detector cannot be operated in close proximity with other metal detectors because the transmit and receive timing circuitry of the individual metal detectors will be operating at relatively different positions in given timing cycles for respective detectors. For example, FIG. 3 shows three separate metal detectors 301, 302 and 303 each with their own coil operation controlled by circuitry within each respective detector. In other words the transmit/receive timing of individual metal detectors 301, 302 & 303 is unaligned. Taking any two detectors out of detectors 301, 302 or 303 and designating these two example detectors as A and B, with reference to FIG. 1, in the timing sequences shown, at time t, metal detector A has finished transmitting and has started to receive within its timing cycle T1. However, metal detector B is still transmitting at time t in its own characteristic timing cycle. Given that metal detector B is still transmitting whilst metal detector A is entering a period of receiving, metal detector B's transmitted signal will cause interference to the received signal of metal detector A. This disruptive overlap of transmit and receive timing within each respective cycle of the detectors is illustrated in FIG. 3 where the transmit signal 304 from each detector's interrogating coil is more or less simultaneously received by each of the other detectors operating within close proximity.
Another problem identified by the inventor with metal detectors is that the depth of detection offered by a metal detector is limited by the size of the search or primary coil that produces the interrogating magnetic field. Large search coils detect deeper than smaller search coils and therefore a straightforward solution to the problem of providing greater depth or penetration for a metal detector is to use a larger search coil. However, large search coils have their own deficiencies for use in that, for example, with portable field use metal detectors having large search coils they are difficult to use in long grass or in between objects like trees because of their size. There is also the drawback that stationary (non-portable) metal detectors, for example, those for industrial use will require much greater space to occupy because of the required size increase to accommodate larger coils.